1. Field of the Invention
The present invention relates to a toner for developing electrostatic images, and more particularly to a toner for use in direct or indirect electrophotographic developing methods. In addition, the present invention also relates to a toner container containing the toner, a developer including the toner, a method for preparing the toner, and a process cartridge and an image forming apparatus using the toner.
2. Discussion of the Background
Electrophotographic image forming methods have been used for various fields. Electrophotographic image forming methods typically include the following processes.    (1) charging the surface of an image bearing member such as photoreceptors (charging process):    (2) irradiating the charged image bearing member with light to form an electrostatic latent image on the image bearing member (light irradiation process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (development process);    (4) transferring the toner image onto a receiving material fed from a sheet feeding device optionally via an intermediate transfer medium (transfer process);    (5) fixing the toner image to the receiving material upon application of heat and pressure thereto (fixing process); and    (6) removing toner particles remaining on the image bearing member and intermediate transfer medium without being transferred so that the image bearing member and intermediate transfer medium are ready for the next image forming processes (cleaning process).
Pulverization methods are well known as toner preparation methods. Pulverization methods typically include the following processes:    (1) melting and kneading a toner composition including a thermoplastic resin serving as a binder resin, a colorant, an optional additive, etc. upon application of heat thereto (kneading process);    (2) cooling the kneaded toner composition (cooling process);    (3) pulverizing the cooled toner composition (pulverization process); and    (4) classifying the pulverized toner composition to prepare toner particles (classification process).
Toners prepared by such pulverization methods typically have a large average particle diameter, and therefore it is difficult for the toners to produce high quality images.
In attempting to produce high quality images, polymerization methods and emulsifying/dispersing methods have been proposed. Specific examples of the polymerization methods include suspension polymerization methods in which toner components such as monomers, polymerization initiators, colorants and charge controlling agents are dispersed in an aqueous medium including a dispersant to form drops of an oil phase, and then the oil drops are polymerized to prepare toner particles in the aqueous medium; and association methods in which particles obtained by an emulsion or suspension polymerization method are agglomerated and fused to prepare agglomerated and fused particles, resulting in formation of toner particles.
Although toners prepared by such polymerization methods have a relatively small average particle diameter, the polymerization toners have a drawback in that the binder resin is limited to resins obtained by a radical polymerization method. Namely, resins such as polyester resins and epoxy resins which are preferably used as binder resins of color toners cannot be used therefore.
In attempting to remedy the drawback, emulsifying/dispersing methods in which a mixture of toner constituents such as binder resins and colorants is dispersed in an aqueous medium and emulsified to prepare toner particles have been proposed, for example, in published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 05-66600 and 08-211655. By using these emulsifying/dispersing methods, toner particles with a small average particle diameter can be prepared and various resins can be used for the binder resin of the toner particles. However, the emulsifying/dispersing methods have a drawback in that particles with too small particle diameters are produced, resulting in increase of emulsification loss (i.e., increase of the manufacturing costs).
In attempting to remedy the drawback, JP-As 10-020552 and 11-007156 have disclosed emulsion association methods in which particles prepared by an emulsion method using a polyester resin are agglomerated and fused to prepare agglomerated and fused particles, resulting in formation of toner particles. By using these methods, formation of particles with too small particle diameters can be prevented, and thereby emulsification loss can be reduced.
However, toners prepared by the polymerization methods and emulsifying/dispersing methods tend to have spherical forms due to the interfacial tension of the drops prepared in the dispersing process. Spherical toners cause a cleaning problem in that toner particles remaining on the surface of an image bearing member even after an image transfer process cannot be well removed with a cleaning blade because such spherical toner particles tend to enter the gap between the tip of a cleaning blade and the surface of the image bearing member.
In attempting to solve the cleaning problem, JP-A 62-266550 discloses a technique in that high speed agitation is performed on a dispersion before completion of the polymerization reaction of the dispersion to apply a mechanical force to the particles, so that the resultant toner particles have irregular forms. However, by using this technique, another problem such that the dispersion becomes unstable and thereby particles are united tends to occur.
In addition, JP-A 02-51164 discloses a technique in that particles are agglomerated using a polyvinyl alcohol having a specific saponification value as a dispersant to prepare agglomerated particles (i.e., toner particles) having particle diameters of from 5 to 25 μm. However, the toner particles have large particle diameters.
Further, JP-A 2005-49858 also discloses a technique in that a toner composition liquid including a toner composition, and a filler are added to an organic solvent to form toner particles having irregular forms. However, toners including a filler therein have a high viscoelasticity, and thereby the minimum fixable temperature of the toners increases. When a filler is added to a toner so as to be present on the surface of the toner particles, the viscoelasticity of the toner hardly increases. However, when a filler is present on the surface of the toner particles, problems in that a wax (serving as a release agent) included in the toner particles can hardly exude from the toner particles and the binder resin in the toner particles is prevented from melting away at a fixing process, resulting in deterioration of the low temperature fixability of the toner and occurrence of a hot offset phenomenon.
Further, PCT patent application publications Nos. 2003-515795 (WO01/040878), 2006-500605 (WO2004/019138), and 2006-503313 (WO2004/019137), and JP-A 2003-202708 have disclosed to use layered inorganic materials, in which interlayer ions (such as metal cations) are modified with an organic cation, as charge controlling agents of toner. However, toners including such a modified layered inorganic material have a drawback in that the charge stability thereof deteriorates particularly when environmental conditions largely changed, and thereby the image density of the images produced by the toners is seriously varied.
Because of these reasons, a need exists for a toner which can produce high quality images with good fine-dot reproducibility and good color reproducibility and which has a good combination of fixability, transferability, cleanability, transparency and environmental stability.